The present invention relates to a range detector, and more particularly, to an improvement of a range detector of the light projection type which employs a semiconductor position detecting element (hereinafter referred to as PSD).
As is well known, a range detector of the light projection type for measuring a range by projecting light rays onto an object being photographed and receiving light rays reflected by the object with a PSD has been hitherto employed. In a range detector, as shown in FIG. 1, pulsed light rays thrown onto an object 3 being photographed by a light emitting element 1 for projection which emits infrared rays (hereinafter referred to as IRED) are focused by a projecting lens 2 to irradiate the object 3 and light rays reflected by the object 3 form an image by lens 4 on a PSD 5. An image forming position at a distance x from the optical axis of the lens 4 is determined as follows. EQU x=s.multidot.f/l
Where l is a range to the object 3, f is a distance between the lens 4 and the PSD 5 and s is a base length. Assuming that t is a length of the PSD 5 and the optical axis of the lens 4 is in agreement with the center line of the PSD 5, a ratio between signal currents I.sub.1 and I.sub.2 which are generated from signal electrodes at the opposite ends of the PSD 5 when incident light is at the position x is expressed as follows. ##EQU1## Solving the above equation regarding the object range l, it follows that EQU l=(2sf/t)X{(I.sub.1 /I.sub.1 /I.sub.2 +1)/(I.sub.1 /I.sub.2 -1)}(1)
From this, it is possible to obtain the object range l by calculating the signal ratio I.sub.1 /I.sub.2 obtained from the PSD 5.
FIG. 2 shows a relationship between x and I.sub.1 /I.sub.2. When the object 3 is at infinity, projected light rays are parallel to reflected light rays, namely x=0, so that I.sub.1 =I.sub.2 and I.sub.1 /I.sub.2 =1. In addition, since x.gtoreq.1/l from the equation x=s.multidot.f/l, x on the abscissa is proportional to the reciprocal of the object range. The solid lines a and b represent outputs when respective PSDs having different lengths from each other are employed, namely, the lengths of respective PSDs are t' and t and t'&lt;t. From FIG. 2 it is noted that the longer a PSD, the smaller an output signal current ratio I.sub.1 /I.sub.2.
In addition, when the differential output V between both signals I.sub.1 and I.sub.2 is normalized by the total current I.sub.1 +I.sub.2, the following equation is obtained. ##EQU2##
Thus, the normalized differential output V is proportional to an incident light position x and inversely proportional to an object range. Accordingly, it is also possible to detect an object range by obtaining the output V. The relationship between x and V is shown in FIG. 3, which will be detailed later.
On the other hand, in FIG. 1, when a value x exceeds t/2 as an object range l is reduced, light rays reflected by the object 3 do not form an image on the PSD, failing to detect a range. Namely in a range detector of the kind described a detectable close range is limited. Accordingly, when a length t of the PSD 5 is increased so that a value x does not exceed t/2 even when an object range l is reduced, a detectable close range can be made close up to a practically allowable extent. In practice, however, an amount of signal light incident upon the PSD 5 after reflected by the object 3 is very limited and noise components such as circuit noise are further added to output signals, necessitating increasing of a change of an output signal to an object range l. This is shown in FIG. 3.
In FIG. 3, in which the abscissa represents a reciprocal of an object range l and the ordinate represents a signal current ratio I.sub.1 /I.sub.2, the PSD B is longer in length than the PSD A, so that the former is shorter than the latter in a detectable close range.
Both PSDs A and B would have the same amount of scattering in output signals caused by circuit noise at an equal object range. In FIG. 3, A' and A" represent noise components to A, and B' and B" represent noise components to B. In order to judge a range l1, when a value VA is taken as a decision signal in A, a decision width .DELTA.la is caused in a range component due to A' and A" and when a value VB is taken as a decision signal in B, a decision width .DELTA.lb is caused in a range component due to B' and B". Consequently, it will be noted that A is better than B in accuracy because of .DELTA.la&lt;.DELTA.lb. In other words, in order to reduce a detectable close range, when only a length of the PSD is increased, a decision width increases, thus lowering the accuracy.
To solve such problem, it is conceivable to reduce noise components and to this end, it is necessary to increase an amount of light emitted from the IRED, a light receiving area of the receiving lens and thereby an amount of light incident upon a PSD. It involves other problems of increasing cost and space.
On the other hand, in order to eliminate the disadvantages, it is conceivable to use a method of disposing two light emitters for projection with different base lengths and selecting either of them in response to an object range. However, such method also results in an increase in cost and space.